This invention relates in general to airplane instrumentation and more particularly to an instrument for indicating the angle of attack and direction of flight of an airplane.
As a plane flies through the air, the wings must have at least a certain minimum pitch relative to the direction of flight in order to create the lift for supporting the weight of the plane. This pitch or angle between the wing center line and direction of flight is known as the angle of attack. Since a wing develops greater lift at higher speeds, the necessary angle of attack at higher speeds is less than at lower speeds. There is thus an inverse relationship between angle of attack and air speed for any particular plane.
Although angle of attack may not be important to a pilot at high speeds, it becomes critical at slow speeds since every airplane has a certain angle of attack at which it goes into a stall. Every plane has a corresponding stall speed but this will vary depending on the loading, turning and banking conditions at the stall angle. Accordingly, the angle of attack provides a consistent indication of the airplane's lift characteristics regardless of the payload or turning angle of the plane.
Heretofore, airplanes have been provided with instrumentation to indicate air speed and elevation but only the most sophisticated and expensive planes have been provided with means for indicating the angle of attack. These include commercial and high performance jets wherein a small airfoil or wind vane is mounted exteriorially to sense the direction of airflow past the airplane. Rotation of the airfoil adjusts a potentiometer which causes a read-out of the instrument panel.
An inherent problem of such devices is the lack of accuracy. The potentiometer contacts necessarily create a certain amount of drag which influences the position of the airfoil. At slower speeds approaching stall speed, the error induced by such drag may be intolerable.
Another problem associated with existing airplane instrumentation is that although measurements of altitude and rate of change of altitude may be provided, there is nothing that visually indicates to the pilot his actual direction of flight. Since the angle of attack of an airplane varies with speed as explained above, the inclination of the airplane fuselage affords no reliable indication of direction of travel. At slow speeds, the plane may be flying level or descending with the fuselage disposed with its nose considerably higher than at cruising speeds.
Furthermore, many airplane accidents are caused by overshooting or undershooting the runway during landing, and this problem is a direct result of not having an instrument to indicate the direction that the plane is flying. Accordingly, to make a final visual approach onto a runway with existing instrumentation, a pilot can only estimate the correct rate of descent which will direct the airplane along a path from its starting position to the approach end of the runway.